Separator for electric batteries



Feb. 20, 1962 c. HoRoWlTz ET AL 3,022,367

sEPARAToR FOR ELECTRIC BATTERIES Filed April 23. 1958 50 KMnO4 No -40 I O 2O 30 40 50 60 70 8O 90 IOO POLYVINYL ALCOHOL (PVA) 8O 70 60 50 40' 30 20 IO O METHYL CELLULOSE INV ENTORS.'

CA RL HOROW/ TZ MEYER ME /VDE LS OH/V auf@ / AGE/vr `United lStates Patent SEPARATOR FR ELECTRIC BATTERIES Carl Horowitz, Brooklyn, and Meyer Mendelsohn, New York, N.Y., assignors, by mesne assignments, to Yardney International Corp., New York, N.Y., a corporation ofV New York A Filed Apr. 23, 1958, Ser. No. 730,474 14 Claims. (Cl. ISG- 146) The present invention relates to inter-electrode separators for electrochemical cells..

v This application is -a continuation-in-part of our application Ser. No. 469,292, filed November 16, 1954, now abandoned, for Separator for Electric Batteries.

Among materials heretofore proposed for inter-electrode separators are polyvinyl alcohol, methyl cellulose, wood, paper and regenerated cellulose. As lthe standards of performance for batteries were raised, these known separators no longer met all'requirements for mechanical and electrical strength, electrical characteristics and length of life. The prior art has taught that separators could be constructed by placing layer upon layer of the above materials to form composites, in` attempt to meet the more vrigorous requirements. Each layer, however, would then maintain its individual characteristics as against the succeeding layer so that their combined resistance is the sum total of their individual resistances.

The general obiect of this invention is to provide a new type of separator material having low electrolytic resistance.

Another object of this invention is to provide a separator material resistant to oxidation while maintaining high electrolytic conductivity. V

A further object of this invention is to provide a sepatration ofl active electrode materials while having the ability to resist attack by the oxidizing agents participating in the electrochemical reaction, at 4the same time exhibiting the low electrolytic resistance necessary of highrate batteries.

Another object of this invention is to provide a rechargeable cell .with a separator system having electrolytic conductivity and great oxidation resistance.

A more particular object of this invention is to provide a film-forming separator m-aterial with the aforementioned properties and capable of forming a coating on an electrochemically inert support of negligible electrolytic resistance.

We have found, in accordance wi-th this invention, that a separator can be prepared from homogeneous ice , In addition, they possess the ability to adhere firmly to a supporting base, such as, for example, nylon fabric or rator for electrochemical cells which will withstand pene- A mixtures of polyvinyl com-pounds and film-forming, elecl trolyte-swellable. cellulose ethers (including carboxy ethers). The mixtures when cast into films, upon either temporary or permanent supports, produce separators with oxidation resistance as great vas that known to the art and with extremely high electrolytic conductivity not previously associated with oxidation-resistant materials.

Among the vinyl Ycompounds'capable 'of forming homogeneousandruni-form mixtures with the cellulose ethers vare polyvinyl .-alcohol, polyvinyl-methyl-ether/ maleic-acid copolymer (PVM/MA), polyvinylpyrrolidone and its copolymers with suchvsubstances as vinyl alcohol 'and vinylacetate. The mix-tures are all capable of being cast on a glass surface or the like into selfsupportingfilmsor membranes or into interfibrous films upon fibrous supports. Y conductive but electronically resistant.

,Films produced from the foregoing substances, and embodying the present invention, a-re distinguished -by semi-permeability as well as structural uniformity as evidenced by resistance to gasV diffusionand penetration.

The films are all electrolytically .f

felt, glass-Wool mats, cellulosic sheets and fabrics, viscon casings, and webs of rayon and other electrolyte-resistant synthetics.

The film-forming ability of the homogeneous mixture embodying this invention may be further enhanced, if desired, by adding oxidation-resistant plasticizers such as, for example, glycerols or glycols, and/ or anti-oxidants such as M (p-hydroxyphenyl) morpholine marketed under the trademark SoluX.

The invention will be more fully described with reference to the accompanying drawing in which:

FIG. l is a representation of abattery incorporating separators according to this invention; and f- FiG. Q. is a graphic representation of the properties of separator films compounded in accordance with this invention, plotting the electrolytic resistance and the oxidation resistance as a function of the separator composition.

FIG. 1 is a representation of atypical liquid-electrolyte cell embodying this invention, having a casing 11 and a cover 11a with a pair of negative electrodes 12 and a positive electrode 13 within the casing 11. The electrodes are separated from each other by layers of interelectrode separator materials 14 and 15 according to this invention. The electrodes are connected to positive and negative terminals (not shown) by their respective electrode conductors 12a and 13a. The casing is filled with electrolyte to a level 16 above the tops of the electrodes. The electrodes and the separator materials are completely permeated by the electrolyte, which may be acidic, neutral or alkaline.

The separators of this invention may also be used in so-called dry cells, i.e. cells Where the major portion of the liquid electrolyte is absorbed to form conductive pastes. The electrolytes in such cells may also be acidic,

neutral or alkaline, and the choice of the particular separator materral according to this invention is then governed by the solubility of the film-forming composition in the particular electrolyte. Y Among the various types of dry cells there should be included, by way of example but not -for purposes of limitation, mercury cells, m-anganese-dioxide cells, and lead-peroxide cells.

FIG. 2 shows a representative curve for various proportions of films cast from mixtures of polyvinyl alcohol (PVA) and methyl cellulose. The polyvinyl alcohol 4is shown as representative of the general class of polyvinyl compounds encompassed by this invention and is not meant to limit our invention to this compound. The general shape of the curve is the same for each of the different aforementioned polyvinyl compounds with only slight differences in the numerical values. Curve A is a plot of the electrolytic resistance as a function of the methyl-cellulose content of the film. The conductivity, of course, is a reciprocal function of the resistance. Curve B is a similar plot of the permanganate` number as a funotion'of the iilm composition. This perman ganate number is a representation of the amount of potassium permanganate needed to neutralize the reducing substances liberated by one gram of the film in a quantity of electrolyte in which the film has been soaked for 48V hours. The permanganate number provides a measure of relative oxidation resistance of the various materials used for separators. The lower the permanganate number, the greater the .resistance of the material to oxidation. Itwill be apparent from reference to FIG. 2 that pure methyl cellulose has desirable oxidationresis-tance characteristics but has too high an electrolytic resistance for use in batteries designed to supply current at medium and high rates.

YIt will be 4noted that the ratio of methyl cellulose, representing the cellulosic solids, to polyvinyl alcohol, representing the vinylic solids, can safely range between 1:3 and 3:1. Below the above-mentioned lower limit the electrolytic conductivity of the liilms falls rapidly and cells manulac'tured from such materials have intolerably high internal resistance. Similarly, above the upper limit it has been found that the oxidation number of the separator material indicates a low order of oxidation resistance which cannot safely be tolerated in electrochemical cells that must -be stored or stockpiled for any considerable period of time.

Example 1 A viscous liquid is prepared by mixing the following ingredients:

Y Grams Methyl cellulose, 5% vaqueous solution 100 aqueous solution PVM/MA 100 PVA, 10% aqueous solution 40 Die'thylene glycol 6 Solux (anti-oxidant) 0.1

The mixture is stirred until it is completely homogeneous. A thin layer is cast on glass and allowed to dry at room temperature vfor 4 hours in a current of dry air. The lresulting film is Stripped from the glass. A cell using this lilm as inter-electrode separator between a Zinc negative elect-rode and a lsilver positive electrode has substantially greater cycle life than a control cell using an equivalent thickness of cellophane as the separator.

Example 2 The mixture of Example 1 is applied to a .permeable backing such as a fabric by dipping the fabric into the solution, removing the excess by calender rolls `and then drying the permeable support in a current of low-humidity air. The resulting separator, when used in a silverzinc cell, is found to reduce the zinc-penetration rate for such cells.

Example 3 A -rnixture having the following formula:

p lGrams 10% methyl-'cellulose solution A100 A10% PVA lsolution 100 Diethylene glycol 6 is applied to a polyester-fiber Vfabric (Dacron by Du Pont) by dipping. The excess is removed, after Vimpregnation is complete, by means of a blade. Dry cells are prepared using the 'above material in place of 'the conventional starch-impregnated paper. Such cells furnish higher currents with no substantial change in polarization time. In addition, the cell capacity is maintained even after vprolonged storage at elevated temperature.

Example 4 A mixture of the following formula is `applied to Dacron fabric:

Grams 25% methyl-cellulose solution 125 '20% jpolyvinylpyrrolidone Vsolution (average molec- Vulmweight60,000) v75 The fabricl treated as above is used as an inter-electrode Vseparator in a -silver-zinc alkaline battery. The -cell is then subjected 'to a rapid charge/ discharge .regime and Ahas a .much improved cyc-le life over that of conventional separators.

-It -is understood that the above examples have been cited :to demonstrate general techniques and that 'they in no way are intended to limit the linvention Ywith respect to quantities Vor composition.

We claim:

1. Aninterelectrode separator for electrochemical cells, comprising va permeable support coated with a semi-permeable film composed of a homogeneous mixture of Ia polyvinyl compound, in a proportion ranging rbetween 20% and 80% by weight of said film, with methyl cellulose.

2. An interelectrode separator for electrochemical cells, comprising a semi-permeable film composed of a homogeneous mixture of a polyvinyl eeompound, in la proportion rangingvbet-ween l20% yto 80% by weightfof said film, with methyl cellulose.

3. A-n interelectr'ode :separa-tor for lelectrocliemical cells, comprising a semi-permea'lile film fcomp'of'sed of a homogeneous mixturey of polyvinyl alcohol, i-n a `proportion ranging `between 20% 4and 80% by weight of said film, with methyl cellulose.

4. An interelectrode separator for electrochemical cells, comprising a semi-permeable ilm composed of a homogeneous mixture of polyvinyl-methyl-ethermalcicanhydride, in a proportion ranging between'20'% 'and 80% 'by weight of said -lm, with methyl cellulose. Y

5.. An 'interele'c-trode separator Lfor'electrochei'nical cells, comprising a 'semi-permeable film formed of ahomogeneous mixture o'f polyvinyl acetate, in a proportion ranging between 20% 'to 80% Iby Weight of 'said 'lilmwithzmethyl cellulose.

6. An interelectrode separator Vfor electrochemical cells, comprising a semi-permeable tilm formed of 'a homogeneous mixture of `a polyvinylpyrrol'idone copolymer, in a proportion ranging between 20% and '80% `by weight of said film with methyl cellulose.

7. An interelectrode separator for electrochemical cells, comprising a semi-permeable :film composed of a homogeneous mixture -of a polyvinyl compound in a` proportion ranging between '20% and 80% of 'said iilm, `said compound being substantially insoluble injalk'aline solutions, with methyl cellulose.

8. An interelectrode separator ffoi electrochemical cells, comprising a semi-permeable lilm .composed of 'a homo geueous mixture of a :polyvinyl compound, in a proport-ion ranging between 20% and 80% of said il'm, said compound lbeing substantially .insoluble 'in yacidic electrolytes, and methyl cellulose. l

9. An electrochemical cell `comprising at least one negative electrode, at least one positive electrode, an electrolyte vand an interelectrode separator Ib'etween 'said negative and positive electrodes, said separator comprising an electrolyte-permeable film composed of a homogeneous vmixture of a polyvinyl compound, in a proportion ranging between 20% and v'80% Vbyweight `of said iilrn, with methyl cellulose, said cor'npound being 'substantially insoluble insaid electrolyte.

1-0. An 'electrochemical cell according to, lclaim 9, wherein said 'separator further comprises la permeable support for ysaid iilm 'resistant `to said electrolyte.

ll. An lelectrochemical vcell according to Aclaim 10, wherein said support comprises a porous 'polymeric :sheet material.V l

l2. cel-l according to fcllaim 10, wherein "said support comprises a web ofsynthetic iibers.

13. A cell according yto claim l2, wherein said 'fibers consist of nylon. v i

l `14. An intcrelec'trode separator Jforlan electrochemical device comprising -a =sem`ipermeable .lilm .composed of a homogeneous .mixture .of la. polyvinyly compound 1in a .proportion ranging 'between 0% to r80% by weight .of :said ilm Vand methyl cellulose, said A:polyvinyl compound being selectedpffrom vthe Igroup vconsisting :of polyvinyl alcohol, polyvinyl-methyl ether/amaleic-acid compolymel?, plyvinylpyrrolidone, -fpolyvinylpyrrolidonewinpyl alcohol .cof

,polymers yand l:polyvinyl alcohol-Minyl racetate'copolymens.

UNITED STATES PATENTS Koch Apr. 17, 1951 Norris I une 26, 1951 Wilson et a1. Apr. 8, 1952 Yardney et a1 Apr. 14, 1953 5 6 Ellis Ian. 3, 1956 Kirkwood et a1 May 22, 1956 Gosnell et al. June 11, 1957 Herrick et a1. Sept. 30, 1958 

1. AN INTERELECTRODE SEPARATOR FOR ELECTROCHEMICAL CELLS, COMPRISING A PERMEABLE SUPPORT COATED WITH A SEMI-PERMEABLE FILM COMPOSED OF A HOMOGENEOUS MIXTURE OF A POLYVINYL COMPOUND, IN A PROPORTION RANGING BETWEEN 20% AND 80% BY WEIGHT OF SAID FILM, WITH METHYL CELLULOSE. 